Field of the Invention
The invention relates to the field of multilevel power electronic converters, and more particularly to a ride-through and recovery method for DC short circuit faults of a hybrid MMC-based HVDC system.
Description of the Related Art
Modular multilevel converter based high-voltage direct current transmission (MMC-HVDC) system plays an important role in flexible DC transmission for its advantages in system loss, capacity upgrade, electromagnetic compatibility, fault management, etc. However, DC fault is a serious fault type which should be considered in design and operation not only for an offshore MMC-HVDC project using submarine cables, but also for an onshore MMC-HVDC project using overhead lines, and the probability of DC faults is much higher for the latter than it is for the former.
At present, DC faults are processed mainly in the following three ways:
1) Disconnecting an AC system by a device on the AC system side (such as an AC circuit breaker or an AC fuse) which is mostly used in an MMC-HVDC system formed by half-bridge sub-modules. When a DC fault occurs, anti-parallel freewheeling diodes of full-controlled devices in an MMC formed by half-bridge sub-modules connect the AC system to the DC fault point, which means short-circuit of the AC system, and the connection should be disconnected. However, the method features slow response speed, complex timing of restart coordinating actions, and comparatively long system recovery time, and has a great impact on the AC system.
2) Isolating the DC fault by blocking the converters, which is mostly used in an MMC-HVDC system formed by full-bridge sub-modules or by clamping double sub-modules. However, the restart process is slow and complicated, DC fault recovery cannot be realized solely by a converter, non-permanent short circuit faults reoccurring when overhead lines are used as transmission lines cannot be handled, reactive power support cannot be provided for an AC system during failure and active power cannot be transmitted in recovery, stability of a connected AC system is decreased by a suddenly blocking of a converter in failure, and an MMC-HVDC system formed by full-bridge sub-modules or by clamping double sub-modules costs high and an operation loss thereof is large.
3) Isolating a DC fault point by a device on a DC side (such as a DC circuit breaker), a DC circuit breaker features difficult are extinction and line energy dissipation, high production cost, and immature technology, and is rarely used in high-voltage and large-capacity situations.
Although the above three methods may protect a converter station, all of them cannot ensure security of a connected AC system and cannot give support to DC fault recovery.